Waterproofing Under a Stone Countertop

Q.Is it okay to use
Schluter's Kerdi material as a waterproofing membrane
beneath a stone countertop? Because it's flexible, I
plan to use it to cover the plywood substrate on top of
the cabinets, and then turn it up the wall a few inches
so that I can waterproof the area behind the planned
tile backsplash at the same time.

However, if you were planning to install tile on
the countertop instead of the stone slabs, I would
recommend using Schluter's Ditra uncoupling
membrane instead. This is because stone slabs are
basically self-supporting and are not as sensitive
as tile to the inherent dimensional instability of
the plywood deck. Ditra would provide better
lateral flexibility than the Kerdi membrane,
protecting tile and grout from movement in the
plywood substrate caused by changes in moisture
content over time.

You'll need to use a modified thin-set mortar to
bond the Kerdi to the plywood substrate. If there
are any seams, be sure to butt rather than overlap
them, and seal them with Schluter's Kerdi-Band
using unmodified thin-set mortar.